Selective pressures that drive the evolution and maintenance of outcrossing
Morran, Levi, 1981-
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Morran, Levi, 1981-
Although outcrossing is the most widespread mating system among animals and plants, the reason for this prevalence is not fully understood. Evolutionary theory has classified the potential selective pressures driving the evolution and maintenance of outcrossing into two broad categories: deleterious mutations and changing ecological conditions. Despite the inherent advantages of self-fertilization, exposure to either or both of these selective pressures is predicted to favor outcrossing over self-fertilization. I tested these predictions using experimental evolution in populations of Caenorhabditis elegans with genetically modified rates of outcrossing and selfing. I found that outcrossing reduces the fixation of deleterious mutations under mutation influx and that outcrossing expedites adaptation to a bacterial pathogen. Further, I identified facultative outcrossing, a novel life history characteristic, in specific C. elegans strains that predominantly reproduce by selfing but engage in outcrossing when stressed. The shift from a primarily selfing mating system to a predominantly outcrossing system is similar to the environmentally induced facultative sex observed in asexual species, which is thought to enable more rapid adaptation. Facultative outcrossing, although not previously documented, may play a major role in the life histories of many highly selfing species. Finally, most mutations are deleterious and therefore elevated mutation rates are generally thought to produce progressively larger reductions in fitness. Using the chemical mutagen ethylmethanesulfonate, I found the surprising result that populations exposed to a mutation rate at least fifty times greater than natural rates exhibited significantly greater fitness than populations exposed to substantially lower mutation rates. This unexpected fitness optimum may be the result of a volatile balance between the influx of deleterious mutations and compensatory mutations. This work confirms the predictions of several long-standing evolutionary theories by identifying both deleterious mutations and changing ecological conditions as selective pressures capable of driving the evolution and maintenance of outcrossing. These selective pressures, which are ubiquitous in nature, may explain the prevalence of outcrossing relative to selling. This dissertation includes previously published and co-authored materials.